Oriented strand board, also known as OSB, waferboard, Sterling board or Exterior board and SmartPly is a widely used engineered wood product formed by strands (flakes) of wood, often layered in specific orientations. In appearance, it may have a rough and variegated surface with the individual strands (typically around 2.5 cm by 15 cm each) lying unevenly across each other. OSB's are cheap and strong boards, and this makes them excellent building material.
Most countries have certain fire prevention regulations setting certain standards for the fire retardancy of building materials for indoor use, especially for use in public buildings. However, it is a problem with flame retardant agents used in the industry today that they are toxic to humans and animals. Building material treated with these known toxic flame retardant agents will inevitably release some of it into the surrounding environment, which makes building materials that are treated with such toxic agents unsuitable for indoor use.
In WO 03/099633 is disclosed a method of providing flame retardant OSB plates. According to the disclosed method, wood pieces are impregnated with a flame retardant composition before they are processed into strands. The impregnation with the flame retardant is done by the application of a so called vacuum-pressure process, wherein the wood first is subjected to a vacuum; subsequently the flame retardant composition is added and subjected to pressure in order to impregnate the wood pieces. These impregnated wood pieces are than processed into strands that are subsequently used to produce OSB plates in a traditional way.
In WO 01/53621 is disclosed another method of producing flame retardant OSB plates. According to the disclosed method, the strands are led through a sprinkling device, wherein they are sprayed with a watery solution of a flame retardant mineral composition comprising ammonium sulfate and/or ammonium phosphate. Next to the sprinkling device an air heater is placed. The strands are then carried with this hot air through a dryer. From these impregnated strands fire retardant OSB plate are produced.
In WO 97/46635 is disclosed a fire retardant composition for use in OSB plates, said composition comprising a mixture of Ammonium sulfate, Borax and trinatrium phosphate. Furthermore is disclosed an OSB plate comprising such a fire retardant composition, and a method of providing a fire retardant OSB plate. The method involves the steps of impregnating the strands with the above mentioned mixture in a watery solution, and subsequently drying them to a humidity of between 1% to 12%. After this drying, the strands are coated with an adhesive and formed into plates.
In US 2004/0028934 is disclosed a method of fire proofing OSB plates by applying a fire retardant composition to green strands, i.e. strands which have not been pre-dried. The strands are sprayed with the fire retardant composition, or dipped into it. To facilitate an accurate application of the fire retardant, the logs used for the production of strands are weighed before being supplied to a flaker. From this weighing in combination with other parameters, the moisture content of the wood can be estimated.
In WO 03/099533 is disclosed yet another method of providing fire proof OSB plates, according to which microchips are produced from raw wood and impregnated with a flame proofing agent, and shredded into smaller strands after impregnation.
In US 2006/0113513 is disclosed a fire retardant composition and method of its production. The composition comprising water, a highly concentrated alkali, at least one of or a combination thereof, of anhydrous citric acid, citric acid, acetic acid or a related salt thereto. The composition also includes phosphate as well as an alkali metal salt or compound from at least one of lithium, sodium and/or potassium cation in combination with at least one of an acetate, bicarbonate, carbonate and/or hydroxide anion. The composition is then adjusted to a pH value around 6.6% to 7.5% by adjusting the amounts of highly concentrated alkali and/or the citric acid/acetic acid respectively. The compaction is allegedly able to absorb large quantities of heat energy, and is therefore especially suitable as a surface treatment of various objects.
However, in order to produce fire retardant OSB plates in large scale production facilities in an error free and effective manner, it is for example desirable to perform the impregnation of the strands at a pace that will not interfere with, or halt, the running of the rest of the process steps associated with the large scale production of OSB plates. In order to achieve this, it is may be desirable to impregnate the strands with a fire retardant composition at a very high rate and to a sufficient degree—but not to a too large degree, because the impregnation and other processing steps may negatively affect the structure of the strands in such a way that OSB plates produced from them are too weak. Furthermore, some of the fire retardant chemical composition may seep out of the strands and cover its surface, so that the subsequently applied adhesive will not bind the strands property together, which again leads to a useless and weak OSB plate.